We are interested in how osteoblasts, bone-forming cells in the bone marrow, regulate leukemia-engrafting cells in the marrow. Our eventual goal is to elucidate the molecular signals between osteoblasts and acute myeloid leukemia (AML) cells in order to understand biology of leukemia growth and direct novel therapies against it. Osteoblasts are known to regulate the self-renewal and proliferation of hematopoietic stem cells (HSC) in the bone marrow, thus forming a unique microenvironment known as the hematopoietic stem cell "niche". We wish to determine whether this "niche" also regulates leukemia-engrafting cells. To this end, we have created a GFP-labeled AML cell-line -C1498/GFP - that can be transplanted into animals and reproduce the entire leukemia phenotype. First, we will first determine how the kinetics of growth of AML cells is altered in the marrow of genetically manipulated mice (CoM A-PPR) in which osteoblast numbers have been specifically increased. Our preliminary results indicate that the growth of leukemia-engrafting is inhibited by osteoblasts. To further analyse the effects of osteoblasts on leukemia cells, we will purify osteoblasts from marrow using a FACS technique and then co-culture these cells with AML cells to study their growth and differentiation in vitro. Furthermore, we will focus our attention on three potential candidate genes/pathways that might be involved in this regulation. In particular, we are interested in two candidate pathways. Osteopontin is a glycoprotein secreted by osteoblasts that is known to promote quiescence in normal hematopoietic stem cells. We will explore the influence of Opn on AML leukemia cells by using (a) purified Opn on leukemia cells (b) transplanting leukemias into animals lacking Opn (Opn-/-) animals. In addition, we will focus on the Notch signaling pathway to determine its influence on the osteoblast-leukemia interaction. Finally, we will investigate genes involved in the cell-cycle and proliferation of cells, such as p27/kip. Elucidating the interaction between osteoblasts and leukemia cells - thus identifying a niche for malignant cells -will allow us to understand the biology of these tumors and develop novel therapies against leukemia. The regulation of tumor cells by their microenvironment represents an entire novel paradigm in tumor biology.